Optical equalizer for intersymbol interference mitigation

ABSTRACT

A multi-channel optical equalizer for intersymbol interference mitigation compensates for single- or multi-wavelength channels simultaneously and requires few adjustable parameters. The optical equalizer also compensates for overshoots and signal transition degradations in semiconductor optical amplifiers. The equalizer unit uses one control signal for magnitude and one to control signal phase. The equalizer includes a controllable coupling ratio coupler for splitting the light into two portions and a controllable interferometer having two arms, one arm having an additional delay which is equal to an integer multiple of 1/Δf, where Δf is the channel spacing between adjacent wavelengths utilized in the optical system. The controllable interferometer has a controllable delay in a first or second arm for adjusting the relative phase of the light passing therethrough. A coupler combines the two signal portions from the first and second arms to form the equalized output signal. A multistage optical equalizer is implemented using a plurality of optical signal equalizers.

REFERENCE TO PARENT APPLICATION

This is a continuation-in-part of co-pending patent applicationidentified as S. Chandrasekhar 19-31-80-9, Ser. No. 10/735,176, filedDec. 12, 2003 and S. Chandrasekhar 14-29-66-3, Ser. No. 10/393,483,filed Mar. 20, 2003.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to optical equalization arrangementsand, more particularly, to a method and apparatus for implementing anoptical equalizer for intersymbol interference mitigation.

BACKGROUND OF THE INVENTION

Equalizers in the electrical domain to mitigate various impairments indigital optical communication systems are well known. An equalizer inthe optical domain has been proposed but not demonstrated. This proposedequalizer is a multistage lattice filter with many adjustable phaseparameters, the parameters chosen by complicated optimization algorithmsand compensates only one wavelength channel at a time. There have beendemonstrations of single impairment optical compensators, such aschromatic dispersion (CD) compensators and polarization-mode dispersion(PMD) compensators, but these address only one source of impairment at atime.

What is desired is a simple multi-channel optical equalizer forintersymbol interference mitigation which can compensate many wavelengthchannels of a multiwavelength signal simultaneously and which requiresfew adjustable parameters.

SUMMARY OF THE INVENTION

In accordance with the present invention, we disclose a method andapparatus for implementing a less-complex single or multi-channeloptical equalizer for intersymbol interference mitigation which, if sodesired, can compensate many wavelength channels simultaneously andwhich requires fewer adjustable parameters. According to another aspectof our invention, when a semiconductor optical amplifier is usedtogether with our optical equalizer, our optical equalizer cancompensate for overshoots and signal transition degradations of thesemiconductor optical amplifier. Our equalizer method and apparatus isextremely simple, having only two control signals (four in specialcases), and yet can compensate many wavelength channels simultaneously.In one embodiment our equalizer includes a single Mach-Zehnderinterferometer (MZI) with a free-spectral range of 50 GHz (an integermultiple of the channel spacing) and tunable couplers. In electricalequalizer terminology, our equalizer is a single-tap linear equalizer.Our equalizer can dramatically improve the performance ofnon-return-to-zero (NRZ), carrier-suppressed return-to-zero (CSRZ), andpossibly other transmission formats impaired by intersymbolinterference. It can mitigate many impairments simultaneously, includingthose due to transmitter and/or receiver imperfections, filternarrowing, CD, and PMD. Our equalizer is especially dramatic in itsimprovement of the non-return-to-zero (NRZ) amplitude-shift keying (ASK)format, which is significant since this is the least expensive format togenerate and detect.

More particularly, our optical signal equalizer simultaneously equalizesone or more received signals modulated at a preselected modulation bitrate in an optical system, the equalizer comprising

-   a first coupler with a variable coupling ratio for splitting the    light into two or more portions;-   a controllable interferometer unit having two or more arms, each arm    receiving one portion, at least one arm having an additional delay    which is equal to an integer multiple of 1/Δf, where Δf is the    channel spacing between adjacent wavelengths utilized in the optical    system;-   at least one arm having a controllable delay unit for adjusting the    relative phase of the light passing through that arm;-   and a second coupler for combining the signals from the arms.

In another embodiment, the optical signal equalizer for equalizing areceived optical signal modulated at a preselected modulation bit rateis implemented using two or more equalizer units. In yet anotherembodiment, the two or more equalizer units can be implemented to sharecouplers. In a further embodiment, our optical signal equalizer iscoupled to the output of an optical transmitter and adjusted so as tomitigate transmitter output intersymbol interference caused byimperfections in the transmitter.

According to our method of operating an optical equalizer of an opticalsystem, we equalize a received optical signal modulated at a preselectedmodulation bit rate using the steps of:

-   -   splitting the light into two or more variable portions;    -   creating a differential delay between the two or more of the        portions, which is approximately equal to an integer multiple of        1/Δf, where Δf is the channel spacing between adjacent        wavelengths utilized in the optical system,    -   adjusting the phase of the light in one of the two or more        portions,    -   combining the two or more portions into a single output, and    -   whereby the variable adjustments are made so as to mitigate        intersymbol interference impairments in the received optical        signal.

Another embodiment of our invention is directed to an optical signalequalizer coupled between an output of an optical transmitter and anoptical apparatus, a transmitter output signal having a degradationcaused by an imperfection in the transmitter, the equalizer comprising

-   a first coupler with a variable coupling ratio for splitting the    light from the output of the optical transmitter into two or more    portions;-   a controllable interferometer unit having two or more arms, each arm    receiving one portion, at least one arm having an additional delay;-   at least one arm having a controllable delay unit for adjusting the    relative phase of the light passing through that arm, wherein the    variable coupling ratio of the first coupler and the controllable    delay unit are adjusted so as to mitigate said transmitter output    signal degradation caused by the imperfection in the transmitter;    and-   and a second coupler for combining the portions from the arms.

Yet another embodiment of our invention is directed to a multistageoptical signal equalizer of an optical system for equalizing one or morereceived signals modulated at a preselected modulation bit rate, themultistage equalizer comprising

-   a first coupler with a variable coupling ratio for splitting the    light into two or more variable portions;-   a first controllable interferometer unit having two or more arms,    each arm receiving one variable portion, at least one arm having an    additional delay which is equal to an integer multiple of 1/Δf,    where Δf is the channel spacing between adjacent wavelengths    utilized in the optical system, at least one arm of the first    controllable interferometer unit having a controllable delay unit    for adjusting the relative phase of the light passing through that    arm;-   a second coupler for generating two or more variable output portions    that are each a combination of two or more variable input portions    received from the two or more arms of the first controllable    interferometer unit;-   a second controllable interferometer unit having two or more arms,    each arm receiving one variable output portion from the second    coupler, at least one arm having an additional delay which is equal    to an integer multiple of 1/Δf, where Δf is the channel spacing    between adjacent wavelengths utilized in the optical system, at    least one arm of the second controllable interferometer unit having    a controllable delay unit for adjusting the relative phase of the    light passing through that arm; and-   a third coupler for combining the variable portions from the arms of    the second controllable interferometer unit.

According to another operating aspect of our invention, we describe amethod of equalizing one or more received signals modulated at apreselected modulation bit rate in an optical system, the methodcomprising the steps of:

-   (a). splitting the received signal light into a first group of    signals including two or more variable portions;-   (b). creating a differential delay between the two or more of the    variable portions of the first group of signals, which is    approximately is equal to an integer multiple of 1/Δf, where Δf is    the channel spacing between adjacent wavelengths utilized in the    optical system;-   (c). adjusting the phase of the light in one of the two or more    variable portions of the first group of signals;-   (d). generating a second group of signals including two or more    variable portions that are each a combination of the two or more    variable portions of the first group of signals generated in the    phase adjusting step (c);-   (e). creating a differential delay between the two or more of the    variable portions of the second group of signals, which is    approximately is equal to an integer multiple of 1/Δf, where Δf is    the channel spacing between adjacent wavelengths utilized in the    optical system;-   (f). adjusting the phase of the light in one of the two or more    variable portions of the second group of signals; and-   (g). combining the resulting two or more variable portions of the    second group of signals from the phase adjusting step (f).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully appreciated by consideration ofthe following Detailed Description, which should be read in light of theaccompanying drawing in which:

FIG. 1 illustrates a preferred embodiment of our optical signalequalizer for equalizing one or more received signals modulated at apreselected modulation bit rate in a multiwavelength system.

FIGS. 2A and 2B illustrate other embodiments of our optical signalequalizer.

FIG. 3 illustratively shows the effect of our equalizer on receivedoptical signals.

FIG. 4 shows another embodiment of our optical signal equalizerimplemented using a first and second equalizer units.

FIG. 5A shows another arrangement of the two equalizer units of FIG. 4and FIG. 5B shows the transmissivity through one equalizer for differentcoupler drive voltages and a fixed phase voltage.

FIG. 6 shows our equalizer's measured bit-error ratio (BER) versusoptical power for different signal impairments.

FIG. 7 shows our equalizer's measured BER versus optical power fordifferent wavelengths with the same equalizer settings.

FIG. 8A shows an adaptive multiwave optical transmission systemimplemented using our optical equalizer and FIG. 8B shows an applicationof the equalizer in a multiwavelength bus-type architecture.

FIG. 9 shows another arrangement of the equalizer of FIG. 1.

FIG. 10 shows an illustrative optical transmission system including atransmitter, an optical amplifier, our optical equalizer, and areceiver.

FIG. 11 shows an illustrative comparison of two data signals outputtedfrom the optical amplifier one without equalization and the otherequalized using our optical equalizer.

FIG. 12 shows receiver sensitivity for a given bit error rate (BER)versus input power to the optical amplifier measured with and withoutequalization.

FIG. 13 shows an embodiment of our optical signal equalizer used toequalize an optical transmitter output signal.

FIG. 14 shows an embodiment of a multistage optical equalizerimplemented using a plurality of our optical signal equalizers connectedusing two-by-two couplers.

In the following description, identical element designations indifferent figures represent identical elements. Additionally in theelement designations, the first digit refers to the figure in which thatelement is first located (e.g., 101 is first located in FIG. 1).

DETAILED DESCRIPTION

In a digital binary ASK system, performance is measured by the “eye”opening at the decision time. Intersymbol interference (ISI) is thespreading of energy from bit slots into other bit slots, causing eyeclosure. In a digital binary ASK system whose performance is limited byoptical amplifier noise, a significant source of eye closure is due tounwanted signal energy in the “0”s or “spaces” data bit (e.g., 331, 333in FIG. 3) beating with spontaneous emission. If one can improve theextinction ratio of the “0”s (e.g., 371, 373 of FIG. 3), one candramatically improve the performance. In an optically amplified system,any energy in the spaces (“0”s) is much more problematic thanfluctuations in the mark (“1”) level, because of signal-spontaneousemission beat noise. The main purpose of our optical equalizer is toeliminate energy in the spaces (“0”) at the decision time. This is whyan optical equalizer can be much more effective than an electricalequalizer for many kinds of impairments—in the electrical domain at thereceiver it is too late to clean up the spaces to avoid thesignal-spontaneous beat noise.

Our equalizer cleans up the spaces by taking a controllable portion ofthe energy at each time instant (e.g., 341 of FIG. 3) and adding it ±20ps away, which is very close to the bit period for 40 Gb/s signals(e.g., 342 of FIG. 3), with a controllable phase adjustment. Forexample, a low cost transmitter usually has a poor extinction ratio inthe solitary spaces (spaces with adjacent marks). The equalizer can addenergy from the surrounding marks to these with 180° phase, fixing theextinction ratio. As another example, first-order chromatic dispersionhas a symmetric complex impulse response. The equalizer can generate asymmetric complex impulse response that approximately cancels thedispersion.

FIG. 1 illustrates a preferred embodiment of our optical signalequalizer for equalizing one or more received signals 100 modulated at apreselected modulation bit rate in an optical system. Our equalizerincludes a coupler 101 with a variable coupling ratio for splitting thelight into two portions, arms 102 and 103. A controllable interferometermeans 101-105 has two arms 102 and 103, the first arm 102 receiving thefirst of the two portions, the second arm 103 receiving the second ofthe two portions. The first arm 102 having an additional path lengthwhich creates an additional delay that is equal to an integer multipleof 1/Δf, where Δf is the channel spacing between adjacent wavelengthsthat may be utilized by the optical system. Thus, when the opticalsystem is a multiwavelength system Δf is the spacing between channels.When the optical system uses only a single wavelength, e.g., an add/dropmultiplexer, where Δf is the channel spacing between adjacentwavelengths that may be utilized by that add/drop multiplexer. The firstarm 102 has a controllable phase delay 104 for adjusting the relativephase of the light passing through. Note that the controllable phase 104has a range of ±180 degrees and could also have been located in thesecond arm 103. A coupling means 105 then combines the signal portionsfrom the first and second arms to form an equalized output signal 106.By controlling the relative optical signal magnitude 101 and phase 104of the two signal portions in arms 102 and 103, our equalizer improvesthe extinction ratio of the “0”s at the decision point. While coupler101 is shown as having a variable coupling ratio and coupler 105 havinga fixed coupling ratio, coupler 105 could also have a variable couplingratio. Moreover in another embodiment, coupler 105 can have the variablecoupling ratio and coupler 101 have the fixed coupling ratio. Thecoupler 101, coupler 105, and/or the phase delay 104 could be set at thefactory or made adjustable in the specific application.

FIG. 2A illustrates another embodiment of our optical signal equalizer.In this embodiment the only difference over FIG. 1 is that the variablecoupling ratio coupler 101 of FIG. 1 is implemented using a fixed ratiocoupler 201 having two output arms and a controllable transmission unit201A located in either the first arm (as shown) or second arm foradjusting ratio of the magnitudes of the two portions. Thus, fixed ratiocoupler 201 and controllable transmission unit 201A perform the functionof variable coupling ratio coupler 101 of FIG. 1. The embodiment of FIG.1 is preferable to the FIG. 2A embodiment since it has a lower loss.

FIG. 2B illustrates yet another embodiment of our optical signalequalizer. The only difference over FIG. 2A is that the interferometer200 has three arms, with the time delay differences between any two armsof the three being equal to an integer multiple of 1/(the channelspacing in frequency).

With reference to FIG. 3, we can look at the optical signals in the timedomain to understand how our equalizer improves the extinction ratio ofthe “0”s at the decision point. As shown by 310, the input is anon-return-to-zero (NRZ) amplitude-shift keying (ASK) optical datasignal, “010110,” which is transmitted from a transmitter location(e.g., 801 of FIG. 8). Impairments such as an imperfect transmitterand/or receiver, filter narrowing, and CD cause some of the energy fromeach bit (“1” bit) to fall into its neighbors bit (“0” bit). For thisexample, the spilled energy is out of phase with the original signal. Asshown by 320 this signal impairment is 321 and 323. Note that bit 312 isdefined a solitary bit, a bit with both neighbors different from it. Asolitary bit would add energy to both of its adjacent “0” bits, as shownby 321 and 323. The additional energy in “0” bit 323 is the added energyfrom the “1” bit 314. Without equalization, the detector (a square lawdetector) would be decoding a signal that has a power that looks like330, with significant energy in the “0” bit locations 331 and 333. Ourequalizer of FIG. 1 (or equalizer 400 of FIG. 4) has an impulse responseas shown in 340. It generates a controllable amount of energy orcompensation signal 342 (using variable ratio coupler 101) and adds itto the signal at locations +20 ps (Δt bit) away, which is very close tothe bit period for 40 Gb/s signals, with a controllable phase(controllable delay 104).

The controllable magnitude 101 and phase 104 of the equalizer of FIG. 1,are chosen so as to generate an energy signal 342 which is used tocancel out the added energy 323 to the neighboring bits caused bytransmission impairments, especially concentrating on minimizing thepower in the “0”s at the decision point 333. Shown in 350 is the resultof the equalization by the equalizer of FIG. 1. As shown by 356, theequalizer of FIG. 1 will compensate for that portion of the impairmentin the “0” bit, 326, located +20 ps (Δt bit) away from the “1” bit 315.Note however that the equalizer of FIG. 1 only compensates for thatportion of the impairment in the “0” bit, 323, located +20 ps (Δt bit)away, which was caused by solitary bit 311, but does not compensate forthe portion of the impairment in the “0” bit, 353, caused by the “1” bit314. Similarly, the impairment in the “0” bit, 331, located −20 ps (Δtbit) away, which was caused by solitary bit 311, is not compensated bythe equalizer of FIG. 1. It should be noted that a second equalizer 420is needed to compensate for impairments to a “0” bit position which is−20 ps (Δt bit) away (i.e., the impairment caused in “0” bit position323 by “1” bit 314). Such a second equalizer (not shown in FIG. 1) wouldbe connected in series, so that its output connects to the input ofequalizer of FIG. 1. The second equalizer can be implemented and operatein the same manner as the first equalizer 1 of FIG. 1, except thatcouplers 101 and 105 would now be adjusted so as to cause the largestcontribution of the impulse response to come from the longerinterferometer arm. As will be discussed with reference to FIG. 4, thefirst equalizer can also be implemented as shown by 400 and the secondequalizer as shown by 420.

With joint reference to FIGS. 3 and 4, as shown in 360, the secondequalizer 420 takes a controllable amount of energy (using variableratio coupler 423) from bit 362 and adds it to the signal at locations−20 ps (Δt bit) away 361. As will be discussed later, the operation ofthe second equalizer 420 operates in essentially the same manner as theequalizer of FIG. 1 (or equalizer 400 of FIG. 4) except that it affects“0” bit locations −20 ps away rather than +20 ps away. Thus, as shown in370, the second equalizer 420 compensates for the “0” bit impairments351 and 353 to produce the equalized “0” bit shown by 371 and 373,respectively.

As a result, our equalizer provides an especially dramatic improvementin a reduced bit error rate (BER) in the transmission of data signals(because of the more accurate detection of the “0” bits) that utilize anon-return-to-zero (NRZ) amplitude-shift keying (ASK) format. Ourequalizer can also dramatically improve the performance ofcarrier-suppressed return-to-zero (CSRZ), and possibly othertransmission formats impaired by intersymbol interference. It can alsomitigate many impairments simultaneously, including those due totransmitter and/or receiver imperfections, filter narrowing, CD, andPMD.

In summary, using a first and second equalizer (e.g., 400 and 420 ofFIG. 4) the “0” bit impairments 321, 323, and 326 can be minimized to beas shown by 371, 373, and 376, respectively. It should be noted that theabove-described equalization operation of the two equalizers works foran optical system if the channel spacing is equal to N (an integer)times the free spectral range (FSR) (i.e., an integer multiple of 1/Δf,where Δf is the channel spacing) and the data bit rate is essentiallythe same for all channels.

FIG. 4 shows another embodiment of our optical signal equalizerimplemented using a first, 400, and second, 420, equalizer units. Thefirst equalizer 400 includes a first variable ratio coupler 411, acontrollable interferometer 412, and a second variable coupling ratiocoupler 413. The variable ratio coupler 411 includes a fixed ratio(e.g., 50/50) coupler 402 and a controllable variable phase (0±90degrees) element 403. Illustratively, each variable ratio coupler 411and 413 may be implemented as a small Mach-Zehnder interferometers (MZI)(402, 403, 404) with a controllable thermooptic phase shifter 402 in onearm and a quarter-wavelength length increase in the other arm.

The coupler unit 411 includes a fixed ratio coupler 402 to receive anoptical signal at input 401 (note, input 401A is unconnected) and acontrollable variable phase element 403 to control the relative phase ofthe optical signals applied to controllable interferometer 412.Controllable interferometer 412 includes two fixed ratio (e.g., 50/50)couplers 404 and 407 interconnected by two waveguide arms 405 and 405A.The first arm 405 having an additional delay which is equal to aninteger multiple of 1/Δf, where Δf is the channel spacing of thereceived optical signals of the multiwavelength system, the additionaldelay is approximately equal to one modulation bit period. The first arm405 also has a controllable phase shifter 406 (0-360 degrees) foradjusting the phase of 415 with respect to 414. The coupler 407 is usedto recombine the optical signals from arms 405 and 405A. The couplerunit 413 includes a controllable variable phase (0±90 degrees) element408 and a fixed ratio (e.g., 50/50) coupler 402. The variable ratiocoupler 413 receives the optical signal from waveguide arms 405 and 405Aof controllable interferometer 412 and controllable transmission unit408 adjusts the relative phase of the optical signal into coupler 409.Note, the controllable transmission units 403 and 408 together controlmagnitude adjustments in equalizer 400. Additionally, the controllabletransmission units 403 and 408 can both be located in the same or in theopposite arms of coupler units 411 and 413. Coupler 409 combines theoptical signals and the output signal on output 410 is applied to thelower input 421A of second equalizer unit 420. At output 410, the main“1” bit signal is 414 and the “0” bit compensation or satellite signalis 415. The second output 410A of coupler 409 is unconnected.Illustratively, controllable interferometer 412 may be implemented as aMZI (404, 406, 407) with a controllable thermooptic phase shifter 406 inone arm and also has an additional delay which is equal to an integermultiple of 1/Δf.

The implementation of second equalizer 420 is identical to the firstequalizer 400. In equalizer 420 the elements 421 through 433 operate inthe same manner as above-described for elements 401 through 413 of thefirst equalizer 400. However, the inputs 421 and 421A and outputs 430and 430A of second equalizer 420 are connected in a different mannerthan the inputs 401 and 401A and outputs 410 and 410A of the firstequalizer 400. In equalizer 400 the upper input port 401A and upperoutput port 410 are used while in equalizer 420 the lower input port421A and lower output port 430A are used. Thus, in equalizer 420 thelower input 421A connects to the upper output 410 of equalizer 400. Theoutput 430A of equalizer 420 has a main “1” bit signal 414 and both aleading (+Δt bit), 415, and a trailing (−Δt bit), 434, “0” bitcompensation or satellite signal.

Control #1A controls both the controllable phase delay units 403, 408 ofequalizer 400, and Control #1B controls the controllable phase delayunits 423 and 428 of equalizer 420. A Control #2 sets the controllablephase shifter 406 for adjusting the phase of 415. A Control #3, sets thecontrollable phase shifter 426 for adjusting the phase of 434. Theimpulse responses of many realistic impairments are symmetric, so inmany cases Control #1A and Control #1B can be driven with the samevalue, leaving a total of three controls for the equalizer. Thecontrollable phase shifters 406 and 426 may each be implemented using athermooptic phase shifter. Controllable interferometer 412 and 432 canboth be implemented as Mach-Zehnder interferometers (MZIs).

Thus, the equalizer of FIG. 4 may be implemented essentially using twoidentical MZIs in series with tunable couplers, each MZI having afree-spectral range of 50 GHz. Note that the controllableinterferometers 412 and 432 can also be implemented using fixed couplersas in FIG. 2, but with controllable attenuators in the MZI arms. The twoMZIs are connected by a single mode, and so this structure is not alattice structure [3]. Our structure is more straightforward to controland has less polarization and wavelength sensitivity than a latticestructure. In electrical equalizer terminology, our equalizer of FIG. 4is a two-tap linear equalizer. It is designed to compensate 40 Gb/schannels on an integer multiple of 50 GHz grid. It can dramaticallymitigate many intersymbol interference impairments, such as transmitterbandwidth and extinction ratio limitations, filter narrowing, chromaticdispersion, and polarization-mode dispersion. The equalizer isespecially dramatic in its improvement of the non-return-to-zero (NRZ)amplitude-shift keying (ASK) format, which is significant since this isthe least expensive format to generate and detect.

Shown in FIG. 5A is another arrangement of the two equalizer units ofFIG. 4 and FIG. 5B shows the measured transmissivity through oneequalizer for different coupler drive voltages and a fixed phasevoltage. As shown in FIG. 5A, each equalizer unit, e.g., 400, has twotunable couplers, 403 and 408, and a thermooptic phase shifter, 406, inone arm. This biasing of the tunable coupler MZI's significantly reducespolarization-dependent loss (PDL). Both couplers in each MZI 400, 420should always be at the same value, in order to minimize insertion loss,so the controls for couplers 403, 408, 423, 428 are connected togetheras Control #1. More generally, the couplers of each of the equalizerunits 400 and 420 can have separate controls, Controls #1A and #1B. Suchan equalizer would then have four control signals, each controllingapproximately one of the following: the amplitude and phase of the leftand right satellite pulses (see 342, 361 of FIG. 3) of the equalizerimpulse response. Furthermore, mainly only higher order chromaticdispersion significant enough to distort a channel will require anasymmetric equalizer impulse response (which would require a differentcontrol signal voltage Control #1A for couplers 403, 408 of equalizer400 and a Control #1B for couplers 423, 428 of equalizer 420). FIG. 4,assumes no significant higher order chromatic dispersion and thus allthe couplers connect to the same voltage source Control #1. Thus we usedonly three control signals Controls #1, #2, and #3 to obtain the resultspresented in the following section. The measured transmissivity throughone of the MZI's for different coupler drive voltages and a fixed phasevoltage is shown in FIG. 5B. The ripple 501 could not be tuned all theway to zero because of imperfect directional coupling ratios. Thefiber-to-fiber insertion loss, including one connector, is 2.0 dB, andthe PDL is <0.5 dB for one stage.

FIG. 9 shows another embodiment of the optical equalizer. In this case,the same interferometer 900 is used twice. The signal passes oncethrough the interferometer 900, passes through a unidirectional element901 such as an optical isolator or circulator, and reenters theinterferometer 900. The first pass will create a satellite in theimpulse response on one side (415) and the second pass will create asatellite on the other side (434). The advantage of this configurationis that only one interferometer is required. The disadvantage is thatthe magnitudes and phases of the impulse response satellites cannot beindividually controlled as in the FIG. 4 embodiment.

Equalizer Results

For an optical transmission systems operating at 40 Gb/s, one often usesreturn-to-zero (RZ) formats, mainly to improve ISI tolerance. Forexample, RZ formats show an increased tolerance to polarization-modedispersion (PMD). However, RZ formats usually require two modulators togenerate them, making RZ transmitters expensive, complex, and lossy.When our equalizer is used in a NRZ ASK system, it can significantlyreduce the ISI impairments of this less expensive format. In fact, wedemonstrate that a single equalizer can dramatically mitigate ISI forall channels simultaneously, without any need for dynamic control.

FIG. 6 shows illustrative performance improvement of our equalizer tocompensate for differential-group-delay (DGD) and polarization-modedispersion (PMD). In FIG. 6, the hollow and filled symbols are withoutand with the equalizer, respectively, and the data is 40 GBs, 2³¹-1pattern. We set a DGD PMD emulator input polarization for the worstperformance and then measured with and without our equalizer. Aftersetting the equalizer for mitigating the worst-case DGD, we then changedthe input polarization, finding that the BER never became worse. Tofurther check this, we inserted a polarization scrambler before theemulator and measured the average BER, shown as the diamonds in FIG. 6.Thus this equalizer can be used as a PMD mitigator without feedback.Note that the equalizer is not compensating PMD in the true sense—it issimply sharpening the pulses, improving the PMD tolerance, in a mannersimilar to employing RZ formats.

Our equalizer can simultaneously mitigate the effects of threeimpairments, filtering, CD, and PMD, in different sets (not shown). Inthe case of all three impairments the equalizer took a signal with ahigh error floor and made it error-free at a reasonable sensitivity.

To verify that the equalizer can mitigate many channels simultaneously,we launched 16 channels, 100-GHz spaced, on the ITU grid, through +95ps/nm CD and a Gaussian-passband demultiplexer. We optimized theequalizer for ch. 10, and then measured the performance with and withoutthe equalizer, not changing any of the equalizer controls, for everythird channel. As one can see from FIG. 7, the equalizer simultaneouslyimproves all channels. FIG. 7 shows the measured bit-error ratio (BER)vs. optical power for different wavelengths with the same equalizersettings. There were 95 ps/nm CD and a Gaussian-passband demultiplexerin the line.

Shown in FIG. 8A is an application of our equalizer as an adaptiveequalizer for a multiwavelength transmission system. In FIG. 8A, aplurality of wavelengths from transmitters 801 are multiplexed togetherin multiplexer 802 and sent over optical path 803. At the receiverlocation, our adaptive equalizer 804 equalizes all of the receivedmultiplexed channels. The output from equalizer 804 is demultiplexed indemultiplexer 805. An error detector 806 is connected to receive one ofthe demultiplexed wavelengths outputted to one of the receiver channels807. The bit error rate (BER) from error detector 806 is sent over path808 to control unit 809. Control unit 809 generates magnitude and aphase control signals that are applied to equalizer 804. The feedbackloop from the error detector 806, to control unit 808 and to equalizer804, enables the equalizer arrangement to adaptively minimize the errorrate of each of the channels of the transmission system of FIG. 8A.Illustratively, control unit 808 may first try to increase the magnitudecontrol signal to the equalizer 804, if the BER from detector 806increased, the control unit 808 would then reduce the magnitude signalto reduce the BER, otherwise it would keep increasing the magnitudecontrol signal until the BER increased. This operation would continueuntil the minimum BER is obtained. Note that the phase control signal ismostly needed for chirp control (due to CD or transmitter mostly). Ifthe chirp is less likely to change, the phase control signal may not beneeded, only the magnitude control signal would be used. The equalizercould also be placed between the demultiplexer and receiver (location823). However, we would like to emphasize that our equalizer is alsoeffective in a passive mode, with no dynamic adjustment. In such a casethe equalizer could be placed between the transmitter and multiplexer(location 821) or just after the multiplexer (location 822). Note thatin the case that only one channel is passing through the equalizer, itis still advantageous to employ our multiwavelength equalizer becausethen that same equalizer and setting can be used for other channels inthe system, simplifying inventory, for example.

Shown in FIG. 8B is the various locations where our equalizer can beplaced in a multiwavelength bus-type architecture. The multiwavelengthtransmitter 830 is similar to that of FIG. 8A. There are couplers 840,850, etc., along the bus 860 connected to optical filters 841, 851, etc.and receivers 843, 853. Our equalizer (not shown) could be placed in oneof a plurality of locations including locations 861, 862, 863, or 864.In many cases location 862 is preferable since in such a case allwavelength channels can be equalized simultaneously.

Finally, because this equalizer can be made in planar lightwave circuittechnology, it could be integrated with other functions, such as ademultiplexer, add-drop filter, or dynamic gain equalization filter.

Equalization of Nonlinear Distortion

The previous equalization discussion concentrated on intersymbolinterference arising from linear processes. We next consider theequalization of nonlinear distortion from a semiconductor opticalamplifier (SOA).

FIG. 10 shows an illustrative optical transmission system including anoptical transmitter 1001, optical facility 1002, semiconductor opticalamplifier (SOA) 1003, our optical equalizer 1004, optical facility 1005,and an optical receiver 1006. It should be noted that the opticaltransmission system may be implemented in a variety of alternatearrangements to that shown in FIG. 10. For example, if desired anadditional SOA 1007 (note: FIG. 10 says “1107”) can be added betweenequalizer 1004 and optical facility 1005. The SOA 1003 and opticalequalizer 1004 (as well as the second SOA 1007) can be integratedtogether as a common element and can be located at intermediate node ofthe system. Alternatively, the SOA 1003 and optical equalizer 1004 canbe co-located with optical transmitter 1001 at the transmitter node,where the optical transmitter 1001, amplifier 1003, and opticalequalizer 1004 can all be integrated together. Note also that at thetransmitter node the placement of amplifier 1003, and optical equalizer1004 can be reversed. In another embodiment, the SOA 1003 and opticalequalizer 1004 can be co-located with optical receiver 1006 at thereceiver node, where the amplifier 1003, optical equalizer 1004, andoptical transmitter 1001 can all be integrated together. Note that atthe receiver node the amplifier 1003 will generally be located in frontof the optical equalizer 1004, although the order can be reversed ifdesired.

In an illustrative optical system including an optical transmitter(1001), SOA 1003, and an optical receiver, the performance was comparedwith and without the use of our equalizer. The SOA 1003 was acommercially available SOA with a fiber-to-fiber small-signal gain of 18dB at 1550 nm at a bias current of 200 mA. The optical transmitterincluded an external cavity laser at 193.4 THz (1550.12 nm) and achirp-free LiNbO₃ modulator that was modulated using anon-return-to-zero (NRZ) data signal at 40-Gb/s. The modulated opticalwas launched into the SOA at an input power that varied from −3 dBm to−12 dBm to study the degradation of the BER resulting from thenonlinearity of the SOA. For a given BER of 10⁻⁹, our equalizer provideda 5-dB improvement in receiver sensitivity for a pseudo-random bitstream (PRBS) of length 2 ³¹-1. FIG. 11 shows the receiver sensitivityboth with and without equalization for a BER of 10⁻⁹ at different inputpowers to the SOA.

Our equalizer reduced the overshoots on the ‘1’ bits on the risingedges, i.e., the transitions from ‘0’ to ‘1’. This can be seen in FIG.12, which shows the effect of the equalizer (dotted line) on somedistorted data patterns after the SOA using the fast photo-detector. InFIG. 12, the equalizer mitigates the overshoots in the ‘1’ bits, 1201,sharpens the transitions from ‘1’ to ‘0’, 1202, and can flatten a longstring of ‘1’ bits in the NRZ stream, 1203. The overshoots, 1201, fromthe SOA leads to an increase in the average power of the data-stream,resulting in a power penalty. Thus, the use of our equalizer 1004reduces optical signal degradations characterized by these overshootsand signal transitions. However, this improvement is small compared tothe fact that the overshoots on the rising edges can cause a significantenhancement of inter-symbol interference, ISI, in the receiver. This isbecause generally the receiver bandwidth is 0.7 times the bit rate, inorder to optimize its performance in the presence of noise. Thiselectrical filtering causes any overshoots to spill into neighboring bitslots. Thus the main benefit of the equalizer in this case is thereduction of overshoots which then reduced the penalty caused byelectrical ISI in the receiver.

Single-Channel Transmitter-Impairment Mitigation

With reference to FIG. 13, there is shown an embodiment of our opticalsignal equalizer 1302 coupled between an output of an opticaltransmitter 1301 and an optical apparatus (an optical path or device).The equalizer 1302 is used to equalize a transmitter output signalhaving degradation (or distortion) caused by an imperfection in thetransmitter. The transmitter output signal degradations are oftencharacterized by signal overshoots and slow signal transitions. Commontransmitter imperfections are a modulation bandwidth that is too low forthe signal symbol rate or an extinction ratio that is too small. Theequalizer 1302 may be implemented using the arrangements shown as FIGS.1 and 2. In such an arrangement, the variable coupler (101 of FIG. 1)controls the splitting the light into two or more variable portions andthe phase unit or controllable delay unit (104 of FIG. 1) is used toadjust the relative phase of the light passing through that arm of theinterferometer. The phase unit and/or the variable coupler are/isadjusted so as to mitigate said transmitter output signal degradationscaused by the imperfection in the transmitter. Optimum degradationreduction may involve iterative adjustments of the phase unit 104 andvariable coupler 101.

The transmitter and optical equalizer may be monolithically integratedtogether. For example, an InGaAsP-based electro-absorption-modulatedlaser may be monolithically integrated with an equalizer. In this way,one can improve the process yield by using the equalizer to “clean up”imperfections in the electro-absoprtion modulator.

Enhanced Equalizer Using Coherently Connected Interferometers

With reference to FIG. 14, there is shown an embodiment of a multistageoptical equalizer implemented using a plurality of our optical signalequalizers. In a two-stage embodiment 1400 of the multistage equalizer,the first stage equalizer 1401 may be implemented using the arrangementsshown as FIGS. 1 and 2A, except that the output coupler (105 of FIG. 1)is implemented as a 2×2 coupler 1404. The first stage equalizer 1401thus includes a 1×2 variable coupler 1402, an adjustable phase unit orcontrollable delay unit 1403 and a 2×2 output coupler 1404. The secondstage equalizer 1405 may also be implemented using the arrangementsshown in FIGS. 1 and 2, except that it shares the 2×2 coupler 1404 withthe first stage equalizer 1401 and that the coupling ratio of 2×2coupler 1404 is made variable. This sharing of a 2×2 coupler issometimes called “coherent connecting” of interferometers. The secondstage equalizer 1405 thus includes the shared variable 2×2 coupler 1404,a phase unit or controllable delay unit 1406 and an output coupler 1407.The output coupler 1407 may be a 2×2 or a 2×1 coupler. For the two-stageembodiment, third stage equalizer 1421 does not exist.

In a three-stage embodiment 1420 of the multistage equalizer, a thirdstage equalizer 1421 is added to the above-described two-stage equalizer1400. In this three-stage embodiment 1420, the output coupler 1407 ofstage two is implemented as a 2×2 variable coupler that is shared withthe third stage equalizer 1421. The third stage equalizer 1421 thusincludes a 2×2 variable coupler 1407, an adjustable phase unit orcontrollable delay unit 1422 and an output coupler 1423. The outputcoupler 1423 may be a 2×2 or a 2×1 coupler.

While the two and three-stage equalizers have been described as beingimplemented using the equalizer designs of FIGS. 1 and 2A, the equalizerembodiment of FIG. 2B can also be used. (yes, it's true. It would bevery complicated device. It would have more impulse responses in thetime domain, making it more flexible in what it can equalize.)

The operation of the two-stage multistage equalizer of FIG. 14 forequalizing one or more received signals modulated at a preselectedmodulation bit rate in an optical system is now described. The receivedsignal light is split by coupler 1402 into a first group of signalsincluding two or more variable portions. The first controllableinterferometer unit 1402-1404 creates a differential delay between thetwo or more of the variable portions on paths 1408 and 1409, which isapproximately is equal to an integer multiple of 1/Δf, where Δf is thechannel spacing between adjacent wavelengths utilized in the opticalsystem. The adjustable phase unit 1403 is used to adjust the phase ofthe light in one of the two or more variable portions, i.e., on path1408. The 2×2 coupler 1404 generates two or more variable outputportions on paths 1410 and 1411 that are each a combination of the twoor more variable portions on paths 1408 and 1409. The secondcontrollable interferometer unit 1406-1407 creates a differential delaybetween the two or more of the variable portions on paths 1411 and 1412,which is approximately equal to an integer multiple of 1/Δf, where Δf isthe channel spacing between adjacent wavelengths utilized in the opticalsystem. The adjustable phase unit 1406 further adjusts the phase of thelight in one of the two or more variable portions on paths 1411 and1412. Coupler 1407 combines the two or more variable portions on paths1411 and 1412. When a third-stage equalizer 1421 is used it operates inthe same manner as described for the second-stage equalizer.

The two-stage multistage equalizer 1400 can be used as a wavelengthde/multiplexer in a similar manner to the wavelength de/multiplexerdescribed in the publication by T. Chiba et al, entitled Chromaticdispersion free Fourier transform-based wavelength splitters fo D-WDM,published in the Fith Optoelectronics and communicatios Conference (OECC2000) Technical Digest, July 2000. In such an arrangement, in the firstinterferometer 1401 the path lengths 1408/1408A and phase unit 1403would introduce an additional delay which is equal to an integermultiple of 1/Δf (where Δf is the channel spacing of the receivedoptical signals of the multiwavelength system) over what is introducedin path length 1409. In the second interferometer 1405 the path lengths1410/1412 and phase unit 1406 would introduce an additional delay whichis equal to an integer multiple of 2/Δf over what is introduced in path1411. The adjustments of couplers 1402 and 1404 and phase units 1403 and1406 would be used to provide both sufficient crosstalk for thede/multiplexer and appropriate equalization for the signals.

1. An optical signal equalizer coupled between an output of an opticaltransmitter and an optical apparatus, a transmitter output signal havinga degradation caused by an imperfection in the transmitter, theequalizer comprising a first coupler with a variable coupling ratio forsplitting the light from the output of the optical transmitter into twoor more portions; a controllable interferometer unit having two or morearms, each arm receiving one portion, at least one arm having anadditional delay; at least one arm having a controllable delay unit foradjusting the relative phase of the light passing through that arm,wherein the variable coupling ratio of the first coupler and thecontrollable delay unit are adjusted so as to mitigate said transmitteroutput signal degradation caused by the imperfection in the transmitter;and and a second coupler for combining the portions from the arms. 2.The optical signal equalizer of claim 1 wherein the transmitterimperfection may be one or more of a group of imperfections including alimited modulation bandwidth, a limited extinction ratio, and a non-flatmodulation transfer function.
 3. A multistage optical signal equalizerof an optical system for equalizing one or more received signalsmodulated at a preselected modulation bit rate, the multistage equalizercomprising a first coupler with a variable coupling ratio for splittingthe light into two or more variable portions; a first controllableinterferometer unit having two or more arms, each arm receiving onevariable portion, at least one arm having an additional delay which isequal to an integer multiple of 1/Δf, where Δf is the channel spacingbetween adjacent wavelengths utilized in the optical system, at leastone arm of the first controllable interferometer unit having acontrollable delay unit for adjusting the relative phase of the lightpassing through that arm; a second coupler for generating two or morevariable output portions that are each a combination of two or morevariable input portions received from the two or more arms of the firstcontrollable interferometer unit; a second controllable interferometerunit having two or more arms, each arm receiving one variable outputportion from the second coupler, at least one arm having an additionaldelay which is equal to an integer multiple of 1/Δf, where Δf is thechannel spacing between adjacent wavelengths utilized in the opticalsystem, at least one arm of the second controllable interferometer unithaving a controllable delay unit for adjusting the relative phase of thelight passing through that arm; and a third coupler for combining thevariable portions from the arms of the second controllableinterferometer unit.
 4. The multistage equalizer of claim 3 wherein thethird coupler has two outputs, to enable the multistage equalizer tofunction as a multiplexer or demultiplexer.
 5. The multistage equalizerof claim 3 arranged as a three-stage equalizer, wherein the thirdcoupler generates two or more variable output portions that are each acombination of portions from the arms of the second controllableinterferometer unit; a third controllable interferometer unit having twoor more arms, each arm receiving one variable output portion from thethird coupler, at least one arm having an additional delay which isequal to an integer multiple of 1/Δf, where Δf is the channel spacingbetween adjacent wavelengths utilized in the optical system, at leastone arm of the third controllable interferometer unit having acontrollable delay unit for adjusting the relative phase of the lightpassing through that arm; and and a fourth coupler for combining thevariable portions from the arms of the third controllable interferometerunit.
 6. The multistage equalizer of claim 5 wherein the third couplerhas two outputs, to enable the multistage equalizer to function asmultiplexer or demultiplexer.
 7. A method of equalizing one or morereceived signals modulated at a preselected modulation bit rate in anoptical system, the method comprising the steps of: (a). splitting thereceived signal light into a first group of signals including two ormore variable portions; (b). creating a differential delay between thetwo or more of the variable portions of the first group of signals,which is approximately is equal to an integer multiple of 1/Δf, where Δfis the channel spacing between adjacent wavelengths utilized in theoptical system; (c). adjusting the phase of the light in one of the twoor more variable portions of the first group of signals; (d). generatinga second group of signals including two or more variable portions thatare each a combination of the two or more variable portions of the firstgroup of signals generated in the phase adjusting step (c); (e).creating a differential delay between the two or more of the variableportions of the second group of signals, which is approximately is equalto an integer multiple of 1/Δf, where Δf is the channel spacing betweenadjacent wavelengths utilized in the optical system; (f). adjusting thephase of the light in one of the two or more variable portions of thesecond group of signals; and (g). combining the resulting two or morevariable portions of the second group of signals from the phaseadjusting step (f).